We investigated how the knowledge and usage of two languages relate to sociocultural adjustment in bilingual adolescent samples from three ethnic groups in Indonesia (214 Javanese, 108 Toraja, and 195 Chinese adolescents; 272 females; M[subscript age] = 14.33 years). We tested a model specifying that the vocabulary knowledge of each language mediates the relation between language usage and sociocultural adjustment (here combining strongly correlated measures of adjustment to the ethnic and national culture). The results revealed the same partial mediation model in all groups; bilingualism is important for sociocultural adjustment in all ethnic groups. There were substantial group differences in ethnic language vocabulary scores, but the correlations between ethnic language usage with sociocultural adjustment were the same across groups. Results also showed that ethnic language usage matters more than ethnic language knowledge, and national language knowledge matters more than ethnic language knowledge for sociocultural adjustment. Moreover, our findings confirm that there is a language shift going on in Indonesia because Bahasa Indonesia as national language, which was the second language in the past, has become the dominant language across ethnic groups in Indonesia.